Field of the Invention
The present invention relates to a method of manufacturing a gear having a plurality of external teeth on an outer circumferential wall surface thereof and a plurality of internal teeth on an inner circumferential wall surface thereof, and a forging apparatus for manufacturing such a gear.
Description of the Related Art
Gears that have external teeth on an outer circumferential wall thereof and internal teeth on an inner circumferential wall thereof are used in some applications. In this case, for example, the internal teeth are held in mesh with teeth on a circumferential side wall of a shaft member, thereby coupling the gear to the shaft member. Therefore, a process of welding the gear to the shaft member is not necessary.
When this type of gear is manufactured, the external teeth and the internal teeth may be formed by different forging apparatus, respectively. Alternatively, the external teeth and the internal teeth may be formed by one forging apparatus, as disclosed in Japanese Patent No. 3769856. Specifically, the forging apparatus disclosed in Japanese Patent No. 3769856 has a mandrel serving as an internal tooth forming die for forming internal teeth and a die serving as an external tooth forming die for forming external teeth.
With the forging apparatus thus constructed, a gear is manufactured as follows. First, after the mandrel is passed through a through hole in a workpiece, an upper die is lowered while the workpiece is being sandwiched by a knockout pin and an outer punch. As a result, the material of the workpiece flows into grooves between internal tooth forming teeth of the mandrel, whereby internal teeth are preformed on an inner circumferential wall of the workpiece.
Then, the upper die and the knockout pin are further lowered. The workpiece is pressed by the outer punch and pushed out downwardly. At this time, external tooth forming teeth on an inner circumferential wall of the die extrudes external teeth (spur gear teeth) on an outer circumferential wall of the workpiece. Since the material of the workpiece also flows radially inwardly, the preformed internal teeth are formed into helical splines as a final tooth form, whereupon the desired gear is produced.